Diffraction compensated mirror for laser amplifier

ABSTRACT

Mirrors used in multi-pass laser amplifiers are compensated for diffraction of electromagnetic radiation, caused by abrupt surface transitions at the mirror edges, by rounding of the mirror edges.

United States Patent McLafferty [451 Dec. 19, 1972 [54] DIFFRACTION-COMPENSATED MIRROR FOR LASER AMPLIFIER [72] Inventor: George II.McLaHerty, Manchester,

Conn.

[73] Assignee: United Aircraft Corporation, East Hartford, Conn.

[22] Filed: Feb. 16, 1970 [21] Appl. No.: 11,717

[52] U.S. Cl. ..330/4.3, 331/945, 350/204, 350/293 [51] Int. Cl ..H0ls3/05 [58] Field of Search....330/4.3; 331/945; 350/160 R, 350/171, 204,293

[56] References Cited UNITED STATES PATENTS 3,433,556 3/1969 Giesecke350/2 04 3,365,671 l/1960 Kogelnik ..330/4.3

OTHER PUBLICATIONS A. I. ialkovskii, Open Resonators Formed by FlatReflectors with Impedance Discontinuity at the Edges 12/66, Pg. 807-812,Soviet Physics.

Pittsburgh Plate Glass Co., Various Edge Finishes" 1946, Section F.8 &F.3.

Primary Examiner-Samuel Feinberg Assistant Examiner-N. MoskowitzAttorney-Melvin Pearson Williams [57 ABSTRACT Mirrors used in multi-passlaser amplifiers are compensated for diffraction of electromagneticradiation, caused by abrupt surface transitions at the mirror edges, byrounding of the mirror edges.

2 Claims, 2 Drawing Figures DKFFRAQTEON CQMIPENSATEHE MHEKRQR FOR LASERAMPLlllFllEiii BACKGROUND OF THE INVENTION 1. Field of Invention Thisinvention relates to lasers, and more particularly to an improved mirrorfor laser amplifiers.

2. Description of the Prior Art Recent growth in laser technology hasmade possible production of coherent electromagnetic radiation in thevisible and near visible spectrum, herein referred to as laser light. Anillustrative type of laser is the well known carbon-dioxide gas laserwhereby the carbon dioxide gas is caused to achieve a populationinversion of an upper laser energy state relative to the population of alower laser energy state; the resulting nonequilibrium energydistribution promotes the emission of quanta of electromagneticradiation (such as laser light) from a molecule in such a state, themolecule thereby assuming the lower laser energy state. The quantum oflaser light generated or emitted (referred to as a photon) as a resultof this energy state transition is equivalent to the amount of energydifferential between the two energystates; further, the energy carriedby a single quantum or photon is related to the wavelength of the photonby the well known relationship E h(c/)t) where E the energy carried bythe photon h Planck s constant, and

)t the wavelength of the photon.

The phenomenon described has been found to be advantageously exploitablein carbon dioxide gas; additionally, lasers in which the gas is flowinghave been found to be capable of very high laser light output intensity,high gain, and high efficiency.

To benefit fully from the high laser power capability of modern lasers,such as a flowing carbon dioxide gas laser, a suitable laserconfiguration must be provided. For instance, an oscillatorconfiguration, which initiates the generation of laser light output froma suitably excited lasing medium (such as carbon dioxide) requires thepresence of a very high flux intensity within the laser cavity in whichoscillations occur. In order to sustain oscillation, this flux intensitymust be much greater than the intensity of useful laser light outputwhich may be extracted from the cavity during continuous wave operation.For example, use of an oscillator to generate one hundred watts of laserlight may require an internal flux within the osciliator comparable toas much as ten thousand watts. Additionally, because of the requisitesof the oscillator, characteristics of mirrors which may form a part ofthe oscillator cavity render it extremely difficult to extract largeamounts of power from a cavity. For instance, removal of energy by holecoupling tends to distort the output beam as a result of diffractioneffects; the resulting perturbation of the oscillator modes causes powerconcentration on the mirror which can result in excessive heating anddestruction of the mirror. Alternatively, the use of partiallytransmitting mirrors, although valuable at low power levels, results insufficient absorption of energy so as to overheat and damage thepartialiy transmitting mirrors and to substantially reduce theefficiency of power extraction. Therefore, it is preferred to useamplifier configurations when the extraction of laser energy from anexcited optical gain medium is desired at very high power levels.

A known laser amplifier configuration which is very advantageous insofaras the physics of laser power extraction are concerned, includes a pairof planar mirrors oriented at a slight angle to one another with anoptical gain medium disposed therebetween. This permits an incident beamof electromagnetic radiation to enter and exit the gain medium at anangle to the mirrors, thereby avoiding the necessity of having the beampass through the mirrors. Similarly, a variety of path geometries may beprovided in such amplifiers, including delta multi-pass pathconfigurations. This type of amplifier permits increasing the intensityof a relatively low-intensity beam of incident radiation in successivepasses between the amplifier mirrors utilizing the high saturationintensity of the laser gain medium and thereby extracting a maximumamount of power from the laser gain medium. Additionally, since thespurious radiations can result in a beam of substantial intensity atother than the intended path within the amplifier, significant damage tothe internal surfaces of theapparatus can result therefrom.

However, it has heretofore been difficult, if not impossible, to achieveideal operation of multi-pass amplifiers of the type described withoutincurring internal damage in the lasing apparatus. A paramount problemin such amplifiers, sometimes referred to as self lasing, results fromthe generation of random, spurious unwanted laser beams, sometimes of avery small magnitude, within the active region of the amplifier. Thisresults in depleting the population of the upper energy state of thelasing medium and in a corresponding reinforcement in intensity of thespurious beam. In a gas laser gain medium, such spurious beams result ina larger population of the lasing medium molecules in lower energystates, which states promote the absorption of the laser light energyfrom the desired output beam. Thus, the maximum power output obtainable,the gain and the efficiency of the amplifier may be seriously depletedas a result of spurious radiation created within the amplifier region.One source of spurious radiation of the type described is thediffraction of the electromagnetic radiation which can occur at sharpedges or discontinuities in the surface of the mirrors. For instance, ifthe edges of the mirrors are sharp, the diffraction of either the mainbeam of the radiation which may partially spill over to the edges, orthe diffraction of other spurious signals, can result in constructiveinterference, further resulting in unwanted radiation of substantialmagnitude.

SUMMARY OF THE INVENTION An object of the present invention is toprovide an improved mirror for a laser amplifier.

According to the present invention, the exposed edges of a reflectingsurface of a laser amplifier mirror are rounded over the surfacebeginning at a distance in excess of ten times the wavelength of thelaser light from the mirror edge and terminating at the mirror edge.

The present invention substantially reduces spurious radiation withinthe amplifier region of a laser amplifier and it reduces the damagecaused by spurious laser beams internal to the amplifier apparatus. TheinvenlOGOll 0709 tion promotes efficient operation of high power,multipass laser amplifiers. I

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of a preferred embodiment thereof, as illustratedin the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified, sectioned planview of a multipass laser amplifier in accordance with the prior art;and

FIG. 2 is a simplified, sectioned plan view of one embodiment of amulti-pass laser amplifier in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT A typical multi-pass amplifierof the type known to the art is illustrated in FIG. 1. Therein, a pairof mirrors 10, 12 are disposed in closeproximity about a region 14 inwhich a laser gain medium is present. The laser gain medium may comprisea suitable gas, such as carbon' dioxide, flowing between the mirrors l0,12 through the region 14, as illustrated in FIG. 1 by an arrow 16. InFIG. 1, the pathof amplification is indicated by the dashed line 18, andincludes a beam of incident radiation 20 and an amplified output beam22. As viewed in FIG. 1, the surface of each of the mirrors l0, 12 isseen to include a pair of discontinuities, such as the corners 24-27thereof, respectively. As is known in the art, sharp discontinuities inthe surface upon which electromagnetic radiation is incident may resultin diffraction, which in turn can result in both destructive andconstructive interference of various waves generated as a result of thisphenomenon. The result is as if a plurality of sources ofelectromagnetic radiation having various directions of propagation iscreated at the discontinuity. The present invention is directed to theelimination of spurious beams being diffracted from sharp mirror edges.

Referring to FIG. 2, a preferred embodiment of the present inventioncomprises a muIti-pass laser amplifier including a pair of mirrors 30,32, including the improvement of the present invention which comprisesthe provision of rounded corners 35, 37. The mirrors are rounded fromthe edges back along the reflecting surfaces by a distance 40 indicatedin FIG. 2 as D. In accordance with the invention, it has been found thatspurious generation of electromagnetic radiation is substantiallyreduced in a laser amplifier of the type described when the distance Dis equal to or greater than ten times the wavelength of the laserradiation being extracted from the lasing medium within the amplifier.

The edges 35, 37 specially prepared in accordance with the presentinvention minimize the origination of spurious radiation as a result ofdiffraction at sharp edges, prevent the constructive interference ofradiation from spurious sources, and avoid build up of substantial beamsof radiation from unwanted sources. The mirror edges 24a and 26a mayremain sharp (as in the prior art).

The embodiment of the invention illustrated in FIG. 2 and describedherein is simplified, bein shown in schematic form, since the remainingstruc ure of the amplifier forms no part of the present invention. Itsuffices that the mirrors 30, 32 be receptive to incident radiation andcapable of providing amplified output radiation, and a suitable lasergain medium be contained in the region 14 therebetween. Thus, althoughdescribed herein with respect to a flowing carbon dioxide gas laser, anyother suitable lasing medium disposed between the mirrors 30, 32 issuitable in a laser amplifier employing the present invention.Additionally, although the invention has been shown and described withrespect to but one preferred embodiment thereof, it should be obvious tothose skilled in the art that the foregoing and various other changesand omissions in the form and detail thereof may be made therein withoutdeparting from the spirit and the scope of the invention.

Having thus described a typical embodiment of my invention, that which Iclaim as new and desire to secure by Letters Patent of the United Statesis:

1. In a laser amplifier having a pair of substantially planar reflectionsurfaces divergingly disposed on opposite sides of a laser gain regionto redirect laser radiation through said region, the improvementcomprising:

one edge of each mirror being rounded along the plane of the reflectionsurface of the mirror a distance D equal to at least ten times thewavelength of the radiation whereby spurious radiation as a result ofdiffraction at the rounded edge is reduced, said rounded edge being atthe most divergent end of each mirror and the distance D being solimited in length that the reflection characteristics of the mirror areessentially those of a planar mirror as opposed to those of a curved ornonplanar mirror.

2. A laser amplifier comprising:

a pair of mirrors, each having a substantially planar reflectingsurface, said mirrors forming surfaces to reflect the electromagneticradiation in the amplifier, said mirrors adapted to be disposed inproximity to one another with a laser gain medium disposed therebetween,said mirrors being disposed relative to one another so that the majorplane of one of said substantially planar reflecting surfaces is at aslight angle to the major plane to the other of said substantiallyplanar reflecting surfaces, -whereby said surfaces diverge toward oneend of the amplifier region, each of said planar reflecting surfacesterminating in a round surface which extends along the related majorplane a distance which is substantially equal to and not less than tentimes the wavelength of the radiation therein, at the ends of saidsurfaces near the divergent end of said amplifier whereby spuriousradiation as a result of diffraction at the rounded edges is reduced.

I Q! i i

1. In a laser amplifier having a pair of substantially planar reflectionsurfaces divergingly disposed on opposite sides of a laser gain regionto redirect laser radiation through said region, the improvementcomprising: one Edge of each mirror being rounded along the plane of thereflection surface of the mirror a distance D equal to at least tentimes the wavelength of the radiation whereby spurious radiation as aresult of diffraction at the rounded edge is reduced, said rounded edgebeing at the most divergent end of each mirror and the distance D beingso limited in length that the reflection characteristics of the mirrorare essentially those of a planar mirror as opposed to those of a curvedor nonplanar mirror.
 2. A laser amplifier comprising: a pair of mirrors,each having a substantially planar reflecting surface, said mirrorsforming surfaces to reflect the electromagnetic radiation in theamplifier, said mirrors adapted to be disposed in proximity to oneanother with a laser gain medium disposed therebetween, said mirrorsbeing disposed relative to one another so that the major plane of one ofsaid substantially planar reflecting surfaces is at a slight angle tothe major plane to the other of said substantially planar reflectingsurfaces, whereby said surfaces diverge toward one end of the amplifierregion, each of said planar reflecting surfaces terminating in a roundsurface which extends along the related major plane a distance which issubstantially equal to and not less than ten times the wavelength of theradiation therein, at the ends of said surfaces near the divergent endof said amplifier whereby spurious radiation as a result of diffractionat the rounded edges is reduced.